High Influenza Incidence and Disease Severity Among Children and Adolescents Aged <18 Years ― United States, 2022–23 Season

During the 2022-23 influenza season, early increases in influenza activity, co-circulation of influenza with other respiratory viruses, and high influenza-associated hospitalization rates, particularly among children and adolescents, were observed. This report describes the 2022-23 influenza season among children and adolescents aged <18 years, including the seasonal severity assessment; estimates of U.S. influenza-associated medical visits, hospitalizations, and deaths; and characteristics of influenza-associated hospitalizations. The 2022-23 influenza season had high severity among children and adolescents compared with thresholds based on previous seasons' influenza-associated outpatient visits, hospitalization rates, and deaths. Nationally, the incidences of influenza-associated outpatient visits and hospitalization for the 2022-23 season were similar for children aged <5 years and higher for children and adolescents aged 5-17 years compared with previous seasons. Peak influenza-associated outpatient and hospitalization activity occurred in late November and early December. Among children and adolescents hospitalized with influenza during the 2022-23 season in hospitals participating in the Influenza Hospitalization Surveillance Network, a lower proportion were vaccinated (18.3%) compared with previous seasons (35.8%-41.8%). Early influenza circulation, before many children and adolescents had been vaccinated, might have contributed to the high hospitalization rates during the 2022-23 season. Among symptomatic hospitalized patients, receipt of influenza antiviral treatment (64.9%) was lower than during pre-COVID-19 pandemic seasons (80.8%-87.1%). CDC recommends that all persons aged ≥6 months without contraindications should receive the annual influenza vaccine, ideally by the end of October.


Introduction
During the 2022-23 season, influenza activity in the United States began in early October, earlier than in most previous seasons, and returned to pre-COVID-19 levels (1).In addition, high pediatric influenza hospitalization rates in the southeast (2), co-circulation of influenza virus with SARS-CoV-2 and respiratory syncytial virus (RSV), and a limited reduction in the availability of the influenza antiviral medication oseltamivir* were observed.Each year, CDC assesses seasonal severity by comparing current season's influenza activity with thresholds based on peak influenza activity in previous seasons (3) and estimates the numbers and rates of influenza-associated medical visits, hospitalizations, and deaths in the United States (4).This report describes the 2022-23 influenza season among children and adolescents, including seasonal severity, estimated incidence, and characteristics of hospitalized patients.This analysis focuses on the 2022-23 influenza season compared with 2016-17 through 2021-22, excluding 2020-21 (during the peak of the COVID-19 pandemic) when influenza activity was minimal.

Methods
CDC classifies each influenza season's severity using three indicators.First, the percentage of all outpatient visits for influenza-like illness (ILI), defined as fever plus cough or sore throat, is obtained from the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) (5).Second, rates of laboratory-confirmed influenza hospitalization † are estimated through the Influenza Hospitalization Surveillance Network (FluSurv-NET) §, ¶ (6).Finally, the percentage of all deaths due to influenza is calculated from National Vital Statistics System death registry data** (5).For each severity indicator, 50th, 90th, and 98th percentile intensity thresholds (ITs) are calculated from a distribution based on the geometric mean of peak weekly values in previous seasons † † (3,7).Seasonal severity is classified as low if at least two of the three indicators peak below IT 50 , and as moderate, high, or very high if at least two of the three indicators peak above IT 50 , IT 90 , or IT 98 , respectively.
The incidence of influenza-associated outpatient visits, hospitalizations, and deaths is estimated each season and is presented in this report as events per 100,000 population.Influenza-associated hospitalizations are estimated by applying the FluSurv-NET hospitalization rates, after adjustment for possible underdetection based on the probability of being tested and diagnostic test sensitivity, ¶ to the U.S. population.To estimate influenza-associated outpatient visits, the ratio of outpatient illnesses to hospitalizations and the proportion of those with ILI who seek care are applied to the hospitalization estimates.To estimate influenza-associated deaths, the ratio of hospitalizations to deaths is applied to the hospitalization estimates (4,8).Pediatric rates are estimated for children aged <5 years and for children and adolescents aged 5-17 years.
Characteristics of influenza-associated hospitalizations, including influenza vaccination status, § § were abstracted from medical charts by trained FluSurv-NET surveillance staff members using a standard case report form.

Results
For children and adolescents, the 2022-23 influenza season was classified as high severity, with the weekly percentage of outpatient visits for ILI, influenza-associated hospitalization rate, and percentage of deaths due to influenza all peaking between IT 90 and IT 98 (Figure 1).The percentage of outpatient visits that were for ILI and the rate of influenza-associated hospitalizations peaked in late November 2022, 3 weeks before the percentage of influenza deaths peaked; deaths remained high for 4 weeks in December 2022.
Nationally, point estimates of the rates of influenza-associated medical visits, hospitalization, and death estimated during the 2022-23 season were higher among children aged <5 years than among children and adolescents aged 5-17 years (Figure 2).However, among children and adolescents aged 5-17 years, rates of influenza-associated medical visits and hospitalizations were higher during 2022-23 than any season since 2016-17.Children aged <5 years had the second highest rates of influenza-associated medical visits (11,443 per 100,000) and hospitalization (119 per 100,000) in 2022-23 since the 2016-17 season.Rates of influenza-associated deaths in 2022-23 were low and consistent with previous seasons: 1.2 per 100,000 among children <5 and 0.5 per 100,000 among children and adolescents aged 5-17 years (Supplementary Figure, https://stacks.cdc.gov/view/cdc/133678).
During October 1, 2022-April 30, 2023, FluSurv-NET identified 2,762 influenza-associated hospitalizations in children and adolescents aged <18 years, 2,108 of which were sampled and had clinical data available.The median age was 5 years (IQR = 2-9 years), 57.4% were male, and 50.5% had an underlying condition, similar to recent seasons (Table ).The most common underlying medical conditions were asthma, neurologic disorders, and obesity.Most (95.4%) infections were with influenza A virus; 80.2% of those subtyped were A(H3N2) and 19.6% were A(H1N1)pdm09.More than one half (57.1%) of the 2022-23 season's total pediatric hospitalizations occurred during October and November 2022, higher than the percentages occurring in October and November in  the 2016-17 through 2021-22 seasons (1.6%-6.8%).Among hospitalized children and adolescents in 2022-23, 18.3% had received an influenza vaccine, compared with 35.8%-41.8% in 2016-17 through 2021-22.The proportion of pediatric patients with respiratory symptoms who received influenza antiviral treatment during their hospitalization in 2022-23 (64.9%) was similar to the proportion in 2021-22 (61.5%), but lower than that during pre-COVID-19 pandemic seasons (80.8%-87.1%).Among all pediatric hospitalizations, the proportions who were admitted to an intensive care unit (18.4%), who required invasive mechanical ventilation (4.7%), or who died in hospital (0.4%) were similar to the proportions during previous influenza seasons.

Discussion
The 2022-23 influenza season was classified as high severity among children and adolescents, the fourth season with that classification since the 2009 influenza A(H1N1) pandemic.Further, all three severity indicators not only surpassed intensity levels for high severity, but the peaks also occurred early in the season (late November and early December) (1).National estimates of the rates of influenza-associated medical visits and hospitalizations were higher than those during most previous seasons for children aged <5 years and children and adolescents aged 5-17 years.This high incidence strained health care systems, particularly with the co-circulation of SARS-CoV-2 and RSV.† † † Among children and adolescents hospitalized with influenza during 2022-23, a substantially lower proportion were vaccinated compared with previous seasons, which could be related to low vaccination coverage in the population, high vaccine effectiveness, or both.The National Immunization Survey § § § estimates that when pediatric influenza-associated hospitalization rates peaked during the week ending November 26, 2022, only 41.9% of children and adolescents aged 6 months-17 years nationwide had received their annual influenza vaccination (compared with 55.1% by the end of the season).Influenza vaccination coverage by the end of November was similar in 2022 and in 2021 (45.0%), but lower than in 2019 (51.9%) and 2020 (49.7%).Preliminary assessments have shown that the 2022-23 influenza vaccine provided moderately strong (68%) protection against pediatric hospitalization.¶ ¶ ¶ The combination of low influenza vaccine coverage early in the season and unusually early influenza activity (57.1% of the season's pediatric hospitalizations occurred by the end of November) likely contributed to the high observed † † † https://emergency.cdc.gov/han/2022/pdf/CDC_HAN_479.pdf

Limitations
The findings in this report are subject to at least five limitations.First, within FluSurv-NET, influenza testing was performed at the clinician's discretion or based on facilitylevel practices, which might affect the observed clinical epidemiology of influenza-associated hospitalizations.Second, severity assessment and incidence estimation adjustments for the frequency of influenza testing and other ratios were based on previous seasons' data and might not reflect current **** https://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm#summary

Summary
What is already known about this topic?
The 2022-23 influenza season began early, coinciding with circulation of other respiratory viruses.High hospitalization rates among children and adolescents were observed.
What is added by this report?
Among children and adolescents aged <18 years, 2022-23 was a high severity influenza season compared with thresholds based on previous seasons' data; influenza-associated medical visits and hospitalizations met or exceeded incidence in previous seasons.
What are the implications for public health practice?
CDC recommends that all persons aged ≥6 months without contraindications should receive the annual seasonal influenza vaccine, ideally by the end of October.
testing practices or health care-seeking behaviors.Third, FluSurv-NET catchment areas cover approximately 9.0% of the U.S. population; characteristics of children and adolescents hospitalized with influenza might not be generalizable to all pediatric hospitalizations in the United States.Fourth, historical data used for the severity assessment might be a suboptimal comparison if recent influenza activity differs from that before the COVID-19 pandemic; classifications of being above or between threshold levels are qualitative and do not reflect statistical differences.Finally, comparisons of rates of influenza-associated outpatients visits, hospitalizations, and deaths across seasons based on point estimates are descriptive and intended to highlight trends, not statistical differences.

Implications for Public Health Practice
The 2022-23 influenza season was classified as high severity for children and adolescents based on influenza-associated outpatient visits, hospitalization rates, and deaths.Among hospitalized children and adolescents with influenza, receipt of influenza vaccine was lower than that during previous seasons, which might have been in part related to most influenza hospitalizations occurring earlier.The proportion of pediatric hospitalizations treated with influenza antiviral medication was lower than in pre-COVID-19 pandemic seasons; prompt antiviral treatment is important for symptomatic patients hospitalized with influenza.All persons aged ≥6 months are recommended by CDC to receive the annual seasonal influenza vaccine, ideally by the end of October.

FIGURE 1 .
FIGURE 1. Percentage of outpatient visits for influenza-like illness (A), influenza-associated hospitalization rates (B), and percentage of all-cause deaths due to influenza (C) among children and adolescents aged <18 years -United States, 2022-23 influenza season*